The present invention relates to a method of producing polymer particles having a narrow particle size distribution, and more particularly to a method of producing polymer particles having a particle size distribution of .+-.25% with respect to each average particle size in an amount of 95 wt. % or more, within the average particle sizes ranging from 1 .mu.m to 100 .mu.m, which method can be utilized, for instance, as spacers employed in liquid crystal display and wet-type developers and dry-type developers for developing latent electrostatic images; as micro spherical lens for optical fiber, optical branch circuit, optical junction circuit, optical multi-channel, optical switch, optical modulator, and optical connector device; as condenser lens for light emission diode; as surface active microsphere for detection of antigens and antibodies; and as fillers for gas-liquid chromatography.
Conventionally, a variety of studies have been conducted on the methods of producing polymer particles having narrow particle size distributions and in fact a number of patent applications have been filed in connection with the production of such polymer particles.
For instance, the suspension polymerization method has been proposed. In this method, liquid particles of a vinyl monomer are formed in water in the presence of an appropriate dispersion stabilizer, and the dispersed vinyl monomer is polymerized by use of an oil-soluble polymerization initiator. In this method, when the polymerization is carried out under normal stirring conditions, the produced polymer adheres to the wall of the reactor and to the blades of the stirrer, so that the polymer particles become as large as several hundred microns to several millimeters in particle size and the particle distribution becomes very broad, because the particle distribution is predominantly controlled by the factor of the probability of separation and combination of the liquid particles formed in the course of polymerization.
In order to obtain polymer particles with a narrow particle size distribution as the countermeasure, several methods have been proposed, for instance, a method of performing bulk polymerization, followed by the suspension polymerization, a method of partly dissolving a polymer in a monomer to provide the dispersion liquid with an appropriate viscosity, followed by the above suspension polymerization, and suspension polymerization methods using a variety of suspension stabilizers having the property of strong surface activity, using finely-divided inorganic particles which are slightly dissolved in water, or using such suspension stabilizers and inorganic particles in combination. These methods, however, are capable of slightly improving the particle size distribution and still yield polymer particles having a broad particle size distribution.
As a method of obtaining polymer particles having small particle sizes, there is known a micro suspension polymerization method. This method utilizes the principle that liquid particles of polymerizable monomers are stabilized in the presence of a suspension stabilizer with high concentration, and polymerization is performed under the conditions that the liquid particles neither separate nor become united. Therefore, when obtaining polymer particles having a narrow particle size distribution, dispersion of the polymerizable monomer in an aqueous medium before initiation of polymerization is always a problem. For that dispersion, mechanical and physical dispersion devices have been proposed, such as a homomixer, a homogenizer, an atomizer, a one-liquid fluid nozzle, a gas-liquid fluid nozzle and an electric emulsifier. These devices are capable of dispersing the polymerizable monomer in the form of finely-divided liquid particles. However, the necessary conditions for obtaining a uniform particle size distribution are so delicate that it is substantially impossible to obtain polymer particles having a narrow particle size distribution. Particularly, in this method, polymerization also takes place in the aqueous phase, since a suspension stabilizer is employed with a relatively high concentration, so that polymer particles having extremely small particle sizes, for instance, particles with a particle size ranging from 0.1 micron to 1 micron are formed and this becomes a problem.
As another method of obtaining polymer particles having small particle sizes, there is known the so-called emulsion polymerization. In this method, since polymerization proceeds through micelles, each polymer particle uniformly grows, so that a dispersion of polymer particles with a highly uniform particle size distribution can be obtained under appropriate polymerization conditions. However, the particle size of the polymer particles produced by this method is so small, for instance, ranging from 0.1 micron to 1 micron, that, in order to obtain polymer particles having particle sizes of several microns, it is necessary to employ the seed emulsion polymerization in which the above-mentioned small polymer particles are used as seeds or core particles and the polymerizable monomer is added thereto in a system in which further formation of the small polymer particles is prohibited, so that each seed is caused to grow. However, these seeds do not grow significantly, so that it is necessary to repeat the above step several times in order to obtain larger polymer particles. Therefore this method has the drawbacks that it is costly and the polymerization process is long.
Japanese Patent Publication No. 57-24369 discloses an improved seed emulsion polymerization in which the seed polymer particles are caused to swell in two steps, thereby increasing the monomer absorption efficiency of the seed particles. Unquestionably, this method is capable of yielding relatively large polymer particles having a uniform particle size distribution. However, the polymerization process is not only complicated, but the polymerization step and the swelling step are also extremely time-consuming, and it is considerably difficult to set up appropriate conditions for polymerization.
As still another conventional method of obtaining polymer particles having small particle sizes, there is known the so-called dispersion polymerization in organic liquid. In this method, polymerization is performed in a system in which a polymerizable monomer is soluble, but the produced polymer is insoluble and separates out. When polymerization is performed in such a system, an adhesive material, a glassy material or a bulky material is formed during or at the end of polymerization, so that it is impossible to obtain a stable polymer dispersion or polymer particles. However, by use of a block copolymer or a graft copolymer in an organic liquid (in which the produced polymer is insoluble) as a dispersion stabilizer, one component of which copolymer is dissoluble in the organic liquid and the other component of which copolymer is soluble in the produced polymer, a stable polymer particle dispersion can be obtained, so that polymer particles can be recovered.
As the organic liquid in which the produced polymer is insoluble, non-polar solvents, for example, aliphatic hydrocarbons, and polar solvents, for example, alcohols having a small number of carbon atoms, can be employed.
Inventions directed to the production of stable dispersions of polymers in organic liquids, in particular, in aliphatic hydrocarbons, are disclosed in a number of Japanese Patent Publications No. 46-16887, No. 46-38246, No. 46-40685 and No. 47-296. Inventions directed to the production of polymers in polar solvents are disclosed, for instance, in Japanese Patent Publications No. 54-2238 and No. 57-46445. All of these inventions are directed to the methods of producing stable polymer dispersions in organic liquids, with the desired particle size being in the range of 1 .mu.m or less, without any particular necessity for controlling the particle size distribution.